Getting the most from solar panels

A solar system's generation is decided on the day it is installed, by the roof, the panels and the sky. The owner's influence is over what each of those units is worth, and for the same roof that varies over a wide range.

A unit used in the house saves the import price. A unit sent to the grid earns the export price, which is typically around half as much. So the ongoing job of a solar owner is to move use into daylight, to sell what cannot be moved for the best price on offer, and to notice when the system has stopped doing what it should.

Where the value is

Without a battery, a typical household uses between a sixth and a third of what its panels make, and exports the rest. Self-consumption, export and whether a battery earns its keep sets out where those figures come from. Each unit shifted from the export column to the home column is worth the gap between the two prices, and at present prices that is something over ten pence a unit.

That gap is the value of moving use into daylight. A household that generates 3,600 kWh a year and moves an extra 500 of them into daytime use gains fifty or sixty pounds a year for no outlay, every year the panels last. Over the twenty-five years or more of a panel's life that is well over a thousand pounds at today's prices, and it needs no equipment.

Moving use into daylight

The appliances that can be moved are the ones that heat something or that run on their own. In rough order of what they use:

Water heating. A house with a hot water cylinder and an immersion heater has the largest movable load in the building. An immersion running at full power draws around 3 kW, which is more than most arrays produce in any hour, so left to itself it pulls from the grid as well as the roof. A solar diverter fixes that by watching the export and feeding the immersion only with the surplus, so the cylinder heats through the afternoon on electricity that would otherwise have left at the export price. For a house that heats water electrically it is usually the first thing worth adding. For a house on gas the sum is closer, since it is replacing gas at a lower unit price, and worth doing with your own figures.

Washing machine, dishwasher, tumble dryer. Most have a delay-start or a scheduled start. Set them to run from late morning. Two of these run in sequence across a summer afternoon can use most of a small array's surplus.

Electric vehicle charging. A car's battery is far larger than a house battery, and a charger that can follow the solar surplus, charging faster when the sun is out and pausing when a cloud goes over, can lift self-consumption a long way on days the car is at home. Even a charger without that feature, set to charge during the day when the car is at home, does most of the job.

Heat pump hot water and cooling. A heat pump's hot water cycle can be scheduled for early afternoon in summer, when the roof is producing and the house needs no heating. A heat pump that can cool can do the same. Both are settings on the heat pump's controller and take a minute to change.

The small always-on loads and the things you use when you use them are not worth moving. Cooking dinner at two in the afternoon to save a few pence is not worth the disruption.

The export tariff

What cannot be used at home is sold, and the price it sells for is not fixed. Export tariffs vary by a factor of several between suppliers, and the rate a system was signed up to when installed is not necessarily the best one available two years later.

Three things to check.

That you are being paid at all. Export payment needs a smart meter reporting export readings half-hourly, an export tariff applied for and set up, and the installation certificate provided to the supplier. Any of the three missing means the export is going out unpaid. A surprising number of systems run for a year or more before the owner notices the export line on the bill is blank.

Whether the rate is still competitive. Compare once a year. Some suppliers pay a higher export rate to customers who also import from them, so the two tariffs are worth considering together, and the combination that pays best for a house with solar may be different from the one that pays best without.

Whether a time-of-day export rate suits you. A few tariffs pay more for export in the early evening. A house with a battery can hold its afternoon surplus and release it in that window. Without a battery, the export happens when the sun shines and the variable rate is of limited use.

If there is a battery

A battery roughly doubles self-consumption, and the tables in the self-consumption guide show the scale. It also adds the charging schedule to the list of things to manage.

In summer the battery should charge from the roof and nothing else. In winter, when the array produces little, a battery on a time-of-day import tariff can charge overnight at the cheap rate and run the house on it through the day. Most battery systems have a setting for this, and some suppliers' tariffs assume it. The schedule needs changing with the seasons, typically twice a year, and again when the tariff changes.

The battery's depth of discharge and the reserve it keeps for backup are set on installation. A high reserve protects against a power cut at the cost of capacity that could have been used every day. A reserve of 20 per cent in a house that never has power cuts is a fifth of the battery doing nothing, and it can be lowered in the settings.

Watching the figures

Every inverter records what it generates and most report it to an app. The single most useful thing an owner can do is look at the annual total and compare it with the estimate on the quotation, which was calculated by the method in the installation standard from the roof's orientation, pitch, shading and location.

A real year will not match the estimate. Sunshine varies by around ten per cent from year to year, and the standard's figures are long-run averages. A system that makes 90 to 110 per cent of its estimate is doing what it should. One that makes 70 per cent of it has a problem, and the sections below cover the likely ones.

Beyond the annual total, a look at the daily curve in the app tells you a good deal. A clear summer day should give a smooth hump peaking around noon. If one side of the hump has a bite out of it, something is shading the array at that time of day. A flat top means the inverter is clipping, which is normal on a system with more panel than inverter and only worth worrying about if it happens most days. If the trace drops to zero and stays there, there is a fault.

Shade changes over the years

The shade assessment on the quotation was done on one day and describes the trees as they were then, and they have been growing since. A birch that was below the roofline at installation can take a noticeable share of the output by year seven, and because of how panels are wired together the loss is larger than the shadow. Shading, and why it costs more than it looks explains the mechanism.

Stand where you can see the array on a clear day at mid-morning, noon and mid-afternoon, once in early summer and once in winter when the sun is low. A shadow that was not there at commissioning is a reason to look at the generation figures for that time of day, and if it is your own tree, to consider whether it wants cutting back.

Cleaning, and whether it is worth it

Rain washes panels at most British pitches, and the loss from ordinary dust and pollen is a few per cent at worst. Cleaning pays on a low pitch that does not shed water, on a roof under trees that drop sap or that birds sit above, and on a coastal site where salt builds up. On any roof, the strip along the bottom edge of a panel is where dirt collects against the frame, and a band of dirt there can shade the bottom row of cells.

If cleaning is done, it is done from the ground with a soft brush on a pole and clean water, or by somebody with the equipment to be on the roof safely. A pressure washer can force water past the seals, detergents leave a film, and cold water on a hot panel in full sun can crack the glass, so all three are best avoided.

The inverter

The inverter is the component that fails, and What wears out, and when gives the expected life. Before it fails it usually gives warning, as a fault code on the display, generation reading zero on a bright day, or a fan that has become noticeably louder. A fault code in June is worth acting on within days, since the lost generation adds up quickly at that time of year.

Two things keep it running longer. The first is ventilation. An inverter in a loft with no airflow runs hot, and heat shortens the life of the capacitors inside it, so a loft vent in a sealed space is a cheap change. The second is keeping the space around it clear, so that the display can be seen and checked.

If the inverter is replaced, its output rating should match the array, and the network operator generally needs telling again, since the original notification named a specific device. Telling the network operator covers what that involves.

The annual check

Solar systems need little maintenance and no standard requires an annual service. What is worth doing once a year, ideally in spring before the productive months:

Item	Who
Compare the year's generation with the quotation estimate	Owner
Look at the daily curve on a clear day for shading or clipping	Owner
Check the export payment is arriving and the rate is competitive	Owner
Reset the battery and diverter schedules for the season	Owner
Walk round and look for new shade, debris, or a panel that has shifted	Owner
Check the inverter for fault codes, noise and ventilation	Owner
Inspect the roof fixings, cabling and isolators	Electrician, every few years

The last item is the only one that needs somebody on the roof. Fixings loosen and cable ties perish, and a cable that has dropped onto the roof tiles rubs through eventually. Every five years or so, or after severe weather, is a reasonable interval.

When the house changes

The self-consumption figure moves when the household does. Someone starting to work from home, a car charger being fitted, a heat pump replacing a gas boiler, or children leaving all change how much of the roof's output the house can use, and all of them are reasons to revisit the battery question and the tariff. A house that had no case for a battery when the panels went in, because it was empty by day, may have a good one after an electric car arrives.

Adding panels later is possible but is treated as a new installation. It needs the inverter to have capacity for them, the network operator to be told again, and a new certificate for the added part. Because of that, an array is usually sized for how the house is likely to be used over its life, and adding to it later is the more expensive route.